1. Field of the Invention
The present invention relates generally to food processing machinery and to devices for forming dough into tortilla or snack chips and, more specifically, to a device for preventing fragments of "rework" dough (also called scrap or lacing) from contaminating the final product.
2. Description of Related Art
A surprisingly small number of plant species provide the bulk of our food supply. Of chief importance are the cereal grains of which maize (corn) stands as the major New World grain. The great Amerindian civilizations of Central and South America thrived on maize and developed unique methods to consume it. Because corn kernels lack the elastic gluten proteins found in wheat, corn flour cannot be used to produce a leavened loaf. At the same time, dried maize cannot be readily steamed into a palatable staple like rice.
Therefore, the native Americans developed a technique in which they cooked dried maize with lime or other alkali and then ground the kernels to produce a sticky dough known as masa. Traditionally this dough was shaped by hand and baked on a griddle to make tortillas--the native bread of the New World and a central item in traditional Mexican cuisine.
The popularity of tortillas and other comestibles made from corn has continued to increase. Today, special machines called sheeting heads are used to form the masa dough into relatively thin sheets from which tortillas and other food products, such as chips, are cut. The typical sheeting head comprises a pair of parallel counterrotating rollers, a front roller, and a rear roller, rotating on a horizontal axis.
Masa dough is loaded into the top of the sheeting head, and it is drawn or pinched between the rollers thereby being formed into a thin sheet. Because the dough is sticky, it must be physically stripped from the rollers to release the dough sheet.
In many prior art sheeting heads, the stripping is accomplished by a stripper wire in contact with a lower surface of the rear roller. As the dough is extruded between the rollers, the wire peels the dough from the rear roller, leaving the dough sheet adhering only to the front roller.
After the dough has been removed from the rear roller and left on the front roller, rotation of the front roller carries the sheet of dough into contact with a rotary cutter which counterrotates in contact with the front roller. A series of raised ridges on the cutter cut tortillas, chips, or other products from the dough sheet.
Although cut, the dough still adheres to the front roller. As the dough is moved along by the rotation of the front roller, it comes into contact with a front stripper wire which interrupts the contact between the cut dough and the front roller. As the cut dough is stripped from the roller, it falls onto a moving conveyor belt which carries it into an oven or frier for cooking.
The remainder of the dough sheet that surrounds the cut product on the front roller is called "rework" or scrap. This dough is recycled into the hopper to be mixed with fresh dough and re-extruded. Frequently the front sheeting head roller is equipped with a system of grooves holding strip-like bands. The bands serve to retains edges of the rework so that the front roller can carry the rework up and back into the hopper for recycling. The bands are spaced apart, forming lanes on the roller. Depending on the size of the cut product, a given roller may have a greater or lesser number of bands. A particular product pattern requires a particular band layout so as to optimize retention of the rework on the roller.
Stripper wires have long been a source of problems because they wear and may break, necessitating the shutting down of the whole production line. A number of different improvements have been devised to reduce stripper wire breakage and to eliminate some problems with stripper wires. Recently, workers have discovered that a speed differential between the two rollers can cause the dough sheet to transfer completely from the rear roller to the front roller without the use of a stripper wire. If the front roller is operated at a speed of about 1.1 to 3.0 times that of the rear roller, effective transfer of the dough sheet to the front roller can occur without the intervention of a stripper wire. The exact speed differential for optimal transfer depends on the particular consistency of the dough. For many masa products a ratio of about 1.5 between the front and rear rollers seems effective. This improvement reduces the number of stripper wires per sheeting head to one; the front stripper wire or some other stripping device such as doctor blades or focused air streams is still needed to remove the cut product from the front roller.
However, certain problems have become apparent with the differential roller speed system. Generally the greater the differential in speed between the two rollers, the more complete the transfer of the dough sheet to the front roller. This can be critical, since many dough mixtures used to make corn chips and similar products contain rather coarse particles of ground maize. While this did not pose a problem with the older stripper wire system, with the differential speed method it is possible to have partial transfer where some coarse particles fail to transfer properly and remain on the rear roller. This results in a dough sheet on the front roller with "pinholes" where a particle of maize has been pulled out of the sheet and remains attached to the rear roller. These pinholes are visible as cosmetic defects in the finished food product.
Various experiments have been undertaken to cure the pinhole problem. Surface texture of the rollers may have some effect on dough adhesion. Thus, attempts have been made to polish the rear roller in hopes of decreasing the dough's adherence thereto. Thus far this approach has not solved the problem. The most effective way of avoiding pinholes seems to be increasing the speed differential between the rollers. Unfortunately, this tends to create another problem. Paradoxically, although an increase of the speed of the front roller may assure more complete transfer of the dough sheet to the front roller, this same speed increase may also decrease the adherence of the rework to the front roller. This results in fragments of rework peeling off the front roller and falling onto the cut product on the conveyor belt. Thus, one is forced to choose between the cosmetic defects of pinholes in the product and the cosmetic defects of rework fragments in the final product. Furthermore, this problem is sensitive to the exact consistency of the dough. Therefore, a speed differential may work well with one batch of dough and result in rework or pinhole problems with a slightly different dough of a second batch.